1. Field of the Invention
The present invention relates to a color cathode ray tube display device having a compensating function of effectively eliminating the influence of an external magnetic field which acts in a vertical direction on the display screen of the color cathode ray tube, and a method of eliminating the influence of the external magnetic field.
2. Description of the Prior Art
Such a color cathode ray tube (CRT) for displaying a color image, which is also called Braun tube, has been used for television receivers and the like. The CRT displays images in such a manner as to deflect electron beams emitted from electron guns by means of the magnetic field created by a deflection yoke to scan the electron beams on the screen of the CRT to make phosphorous coating on the screen of the large end of the CRT emit light. Thus, the CRT is susceptible to the influence of an external magnetic field. If an external magnetic field such as the earth's magnetic field or the like acts on the CRT significantly, changes in the positions where the electron beams intersect the phosphor dots on the screen of the CRT occur and hence images cannot be displayed correctly.
A degaussing device disposed in a prior art color cathode ray tube can be effective for compensating for a small amount of change in an external magnetic field which acts in a vertical direction on the screen of the color cathode ray tube. However, in the case of the earth magnetic field, since the directions of the vertical components of the earth's magnetic field in the Northern and Southern Hemispheres are perfectly opposite to each other, such a degaussing device cannot be useful. Thus, in the current state of the art, different types of CRTs, for which different designs (designs of exposure) are respectively carried out according to areas where the CRTs will be used, are manufactured. Additional design works for different areas where CRTs are to be used, accompanied by a new design of a CRT and changeovers to incorporate corresponding deflection lenses into CRTs destined for areas where the CRTs are to be used result in an increase in the cost of manufacturing.
When an external magnetic field acts vertically on such a CRT, the positions of electron beams which intersect the screen of the large end of the CRT are moved parallel in a horizontal direction regardless of where the electron beams intersect within the whole screen of the CRT. However, when an adjustment is made to a CRT destined for the Northern Hemisphere by means of a compensating magnetic field used for CRTs destined for the Southern Hemisphere, the horizontal travels of the electron beams differ according to whether they intersect a central region of the screen or an upper or lower region of the screen, which depend on the structure of the degaussing device; the horizontal movement of the electron beams which intersect the upper or lower region of the screen is larger than those of the electron beams which intersect the central part of the screen.
Thus, there remain large horizontal movements of the electron beams within the upper or lower region of the screen of the CRT due to the influence of the external magnetic field when an adjustment is made properly by means of the purity (the degree of a match regarding position between each of the electron beams and each of corresponding phosphor dots) correction function of the CP-ASSY disposed in the prior art CRT and used for the convergence and purity adjustments in such a manner that the electron beams intersect the corresponding phosphor dots in the central part of the screen of the CRT precisely so that the phosphor dots are respectively overlaid with the electron beam spots.
Accordingly, as shown in FIG. 8, in the central part of the screen of the color CRT 1, each of the electron beams 8 impinges on the whole of each of the corresponding phosphor dots so as to make the whole of each of the phosphor dots 9 emit light. In contrast, in the upper or lower region of the screen of the CRT 1, each of the electron beams 8 is shifted considerably and hence impinges on only a part of each of the corresponding phosphor dots 9. Thus, only the part of each of the phosphor dots can emit light. As a result, images cannot be displayed properly. In FIG. 8, displacements of one of the electron beams for the three primary colors, red, green and blue are shown for simplicity.
One presently available mechanism for making a correction to vertical deflections of the electron beams 8, disclosed in Japanese Patent Application Laid Open (KOKAI) No. 3-219537, generates an uniform horizontal bipolar magnetic field in synchronization with the vertical deflection current so as to deflect the electron beams 8 in a vertical direction in addition to the main vertical deflection caused by the vertical deflection current. Thus, the deflection mechanism cannot deflect the electron beams 8 which will intersect the upper or lower region of the screen of the CRT 1 in one identical horizontal direction so as to correct displacements of the electron beams from corresponding phosphor dots 9 on the screen of the CRT.
A disadvantage with the currently available color cathode ray tube display devices constructed as mentioned above is that different designs (designs of exposure) need to be respectively carried out for different types of CRTs in accordance with areas where the CRTs will be used, for example, CRTs destined for the Northern and Southern Hemispheres, so as to manufacture them, and hence an increase in the load on the management of components of CRTs which is accompanied by new designs of different types of CRTs and an increase in the number of types of CRTs and changeovers to incorporate corresponding deflection lenses into CRTs destined for areas where the CRTs are to be used result in an increase in the cost of manufacturing.